A Merck HIV vaccine might have made recipients more susceptible to the deadly virus that causes AIDS.Â Merck had cancelled clinical trials of the HIV vaccine in September, after it became clear that it was not effective against the virus.Â Â Â Â Now, the problems with the Merck HIV vaccine have prompted researchers to warn that other vaccines currently undergoing clinical trials could cause similar side effects for recipients.

Since it cancelled the HIV vaccine clinical trial in September, Merck has been analyzing data from the study.Â All clinical trial participants had been HIV negative at the start of the Merck HIV vaccine trial.Â Â However, the companyâ€™s analysis found that those study participants given the vaccine were more likely to end up with an HIV infection than those who were given a placebo.Â It is highly unlikely that the Merck HIV vaccine would have transmitted the virus itself, as it contained only a few synthetic HIV fragments loaded onto a genetically modified cold virus.Â But researchers are considering the possibility that that the vaccine somehow modified the recipientsâ€™ immune system in a way that made HIV infection more likely.

Among those study participants who were more likely to become infected with HIV where those who had a high level of pre-existing immunity to the cold virus used in the vaccine.Â According to Merck, by mid-October, there were 49 cases of HIV infection among the 914 male volunteers who received the vaccine.Â Of the 922 men who did not receive the HIV vaccine, there were only 33 cases.Â Of the 778 men who had high levels of immunity to the cold virus, there were 21 HIV infections among those who got the vaccine, but only 9 among those who did not.

Now, researchers are worried that other vaccines that are made with a modified cold virus â€“ called an adenovirus â€“ could cause similar problems.Â Already, the discovery concerning the Merck HIV vaccine has prompted the National Institutes of Health (NIH) to stop recruiting recipients for other vaccine clinical trials for diseases like Ebola.Â While the NIH has indicated that it will eventually proceed with these clinical trials, a spokesperson for the agency said that researchers wanted more time to advise study participants of the trialsâ€™ possible risks.

The problems with the Merck HIV vaccine are a big blow to AIDS research, as a vaccine is seen as the only possible way to stop the epidemic which killed 3 million people last year.Â Merck had been working on the project for about a decade, and while several other HIV vaccines have also been unsuccessful, the medical community had placed a great deal of hope in Merckâ€™s version.Â Unlike earlier HIV vaccines, the Merck version focused on using the immune systemâ€™s T-cells to attack and kill the HIV virus.Â News of the problems surrounding the Merck HIV vaccine could also discourage people from participating in future vaccine clinical trials, something that could further hamper vaccine development.